Materials Science Forum Vols. 738-739

Abstract: In this work the effect of grain refinement on the shape memory properties of a Fe-Mn-Si-Cr-Ni-Co-Ti alloy was evaluated using compression tests. In order to refine the microstructure, the samples were heavily deformed by equal channel angular extrusion (ECAE) and then annealed at different temperatures ranging from 450°C to 1050°C. These treatments resulted in the formation of intermetallic precipitates and strengthening of austenitic matrix. The results of compression testes show that the higher degrees of shape recovery (56 % for 4% strain) were achieved by the samples with smaller grain size (12 µm).

Abstract: The results presented here concern two NiTi alloys (near-equiatomic NiTi and Ni-rich alloy) subjected to plastic deformation by compression combined with reversion oscillating torsion. The maximal strain obtained was ε_c = 6.20. Finally the alloys were annealed at the temperature range 300 – 500°C for 1 hour. The structure of the as-prepared alloys was studied with the use of temperature X-ray diffraction and TEM observations. Also the DSC and bend and free recovery ASTM tests were carried out. It was found that the structure consists of a mixture of highly deformed B2 parent phase and B19’ martensite. The TEM studies revealed some amorphous areas in the most strained region of the samples. Annealing at lower temperatures caused formation of nanocrystalline structure that grew to the microcrystalline and finally well-defined polygonized structure in annealed at 500°C specimens. Multi-stage transformation was observed in the annealed at lower temperatures samples.

Abstract: In alloys Ni–39(41)Al–xPt (x = 5,10,15,20 at.%) the alloying by Pt strongly increases the Ms point. There is no direct proportionality between the Pt content and the Ms point increasing. No traces of Ni3Al were found in the alloys. The precipitation of Ni₅Al₃ was observed in Ni–39Al–15Pt alloy after cycling through the temperature range between the room temperature and 800°C. The effect of Pt alloying on the martensitic transformation and high temperature martensitic transformation stability is governed by the competition between the martensitic transformation and formation of the Ni₅Al₃ phase upon the cooling. Pt addition, instead of Ni, can resolve the problem of decomposition processes because the alloy composition is shifted out of the Ni₃Al domain on the phase diagram and it reduces the influence of the Ni₅Al₃ phase on the degradation of martensitic transformation.

Abstract: Results of an experimental research on formation of surface layers from a material with shape memory effect (SME) three-componental structure on the basis of TiNiCu combined method including mechanical activation and a plasma dusting in vacuum are presented. It is shown, that the plasma dusting in vacuum of three-componental system TiNiCu allows to receive surface layers in nanostructure condition practically of any thickness. The structure, chemical and phase structure of blankets TiNiCu provide display SME. The average size of grain of coverings TiNiCu fluctuates within 20-200 nanometers. It is experimentally shown, that superficial modifying TiNiCu raises durability of a steel 45 at multicyclic loading on ~30-45 %, and wear resistance in 2,5 3 times.

Abstract: Processes, occurring on annealing of TiNi alloy, processed by high pressure torsion (HPT), were the focus of this research. Some peaks of heat release were found on heating the deformed samples up to 550 °C. If the initial structure of the alloy was amorphous (after 3.5 turns of HPT), then structural relaxation, crystallization and grain growth occurred on heating. When alloys had a crystalline structure (after rotation by 15 and 90 degrees), recovery, recrystallization and further grain growth took place on heating. It was observed that the TiNi alloy, subjected to 3.5 turns of HPT and subsequent heating up to 550 °C, underwent unusual kinetics of martensitic transformations. The austenite B2 phase transformed to the martensite B19' phase in two ways: B2→R→B19' and B2→ B19'.

Abstract: Transmission electron microscopy and X-ray diffraction were used to study structural changes in the Ti-50.5 at. % Ni alloy upon severe plastic deformation by shear under pressure and subsequent heating. An increase in the degree of deformation leads sequentially to a martensite transformation, twinning of martensite crystals, formation of reorientation bands, development of rotational modes of deformation, formation of a nanocrystalline structure, and finally amorphization. A scheme of the formation of amorphous structure of the alloy during deformation is suggested based on the observed structural changes. It has been found that a reverse martensitic transformation might be one of mechanisms of plastic deformation of the alloy. Therefore, as the degree of deformation increases, first forward and subsequently reverse martensitic transformations can occur. The formation of an amorphous structure starts as the degree of deformation reaches 4.2 (one revolution of Bridgman anvils); at a degree of deformation of 6.8 (5 revolutions of the anvils), the process is virtually completed. The crystallization of the amorphous alloy upon heating starts even at 200°C. However, upon heating up to 300°C (for 0.5-h holdings), the kinetics of crystallization is slow. After annealing at 350°C, the complete crystallization with the formation of a nanocrystalline structure with a grain size of 20-70 nm takes place

Abstract: Results of investigations of structure and phase transformations and properties of the TiNi-based alloys with a shape memory effect (SME) after severe plastic deformation (SPD) by cold rolling, cold drawing, high pressure torsion and subsequent annealing are reported. It is found that the baroelastic effects related to the highly reversible martensitic transformations can occur in alloys, subjected to high pressure. The evolution of fine structure of the alloys into nanocrystalline and then amorphous state during SPD and after subsequent annealing have been studied. The effect of grain size on the martensitic transformations and properties of the alloys is discussed.

Abstract: In this study, we investigated the crack nucleation and growth and propagation on the surface of Y-TZP during isothermal phase transformation by low temperature ageing. Crack initiation and growth on the surface of Y-TZP specimen was dependent on the sintered microstructure, i.e, sintered density, grain size, pore structure, residual stress etc.. In the case of Y-TZP with 2mol % yttria content, phase transformation of tetragonal to monoclinic began on the surface and induced a crack nucleation of specimen at the initial stage of low temperature ageing. Most of cracks in 2Y-TZP by low temperature ageing were firstly formed on the surface of specimens (free surface, weak bonding grains, etc.) where the change of strain free energy for a tetragonal to monoclinic transformation was small, and surface cracks grew into the bulk interior through the grain boundaries.

Abstract: Densification and mechanical properties of dental zirconia ceramics were evaluated by different sintering methods. Y-TZP zirconia block was used in this study. Sintering were performed in electric heating furnace and microwave sintering furnace, and then experimented and analyzed on a change in densification according to the sintering time, a change in densification according to thickness, flexural strength and micro-structure of zirconia specimens. Microwave sintering was very effective in considerable mechanical properties such as flexural strength and bulk density was drastically increased than conventional electric heating method. It is also shown that microwave sintering was faster and more economical than common method to be present in qualities which equal or exceed. It will be important to seek the accurate sintering condition of dental zirconia by microwave sintering method and the continuous research is necessary for the study of relationship between sintering methods and mechanical properties.

Abstract: Superelastic NiTi SMA is the base of endodontic files. The flexibility of these instruments permits the preparation of root canals. Unfortunately the intracanal file separation can occur. To have a good idea of the mechanical behavior of these instruments, we propose in this study the finite elements simulations taking into account the real shape of root canals. This has been possible by using a well adapted model describing all the particularities of superelastic SMA and by using representative limit conditions.